The present invention relates generally to opto-electronic devices, and more particularly, to an apparatus for absorbing stray light that is generated in or received by opto-electronic devices.
The performance of light-emitting and light-receiving opto-electronic devices is compromised by stray light. Simply stated, stray light is light that, from the perspective of opto-electronic device performance, is in the wrong place at the wrong time. In some cases (e.g., lasers, etc.), stray light is generated by the opto-electronic device itself, in others (e.g., optical modulators, etc.), stray light originates from an external source. In either case, stray light causes problems, as the following examples illustrate.
FIG. 1 depicts an illustration of opto-electronic device 100, which comprises vertical cavity surface emitting laser (xe2x80x9cVCSELxe2x80x9d) 102 and photodetector 104 that are co-located on first major surface 108 of substrate 106. The substrate comprises electronic driver circuitry (not depicted) for energizing VCSEL 102. Opto-electronic device 100 also incorporates a heat sink (not depicted) that removes the heat that is generated by the electronic driver circuitry and VCSEL 102. Metal backing 112, which is disposed on second major surface 110 of substrate 106, is used to attach the heat sink to substrate 106.
VCSEL 102 is configured to emit output light 114A away from semiconductor circuitry chip 106 in direction 116. But as a result of design and manufacturing compromises, a portion of the output light from VCSEL 102, ray of stray light 114B, is typically emitted toward substrate 106 in direction 118.
The substrate of an opto-electronic device is often transparent to light. For example, substrate 106 is transparent to light having a wavelength of 1.3 microns, an important telecommunications wavelength, when the substrate is made of silicon. In such a case, stray light 114B that is emitted in direction 118 passes through substrate 106, reflects off of metal backing 112 (e.g., at location 120) and is redirected toward first major surface 108.
At first major surface 108, stray light 114B might be received by parts of opto-electronic device 100 that are light sensitive, such as photodetector 104. If stray light 114B is received and absorbed by photodetector 104, cross-talk between input signal 122 and output signal 114A occurs.
FIG. 2 depicts an illustration of opto-electronic device 200. This opto-electronic device comprises micro-mechanical optical modulator 224, the design and operation of which are well-known in the art. The modulator includes membrane 226 that is supported by supports 228 over first major surface 108 of substrate 106. Cavity 230 is formed in the region between membrane 226 and first major surface 108. In micro-mechanical optical modulator 224, substrate 106 is not an active device.
When actuated, such as by an applied voltage, membrane 226 moves toward substrate 230. As membrane 226 moves, the size of cavity 234 (i.e., the distance or gap between the membrane and first major surface 108) changes. This change alters the reflectivity of modulator 224 and, as such, modulator 224 is capable of modulating reflected light. See, for example, U.S. Pat. No. 5,500,761.
In many of the applications for modulator 224, substrate 106 is transparent to light. For example, substrate 106 is transparent to light having a wavelength of 1.55 microns, another important telecommunications wavelength, when the substrate is made of silicon. To prevent stray light from passing out of substrate 106 and into, for example, an output port (not depicted), metal-backing 112 is advantageously disposed on second major surface 110 of substrate 106. Consequently, when modulator 224 is in a low-reflectivity state, most of light 232A that is received by modulator 224 passes through substrate 106 and is reflected off metal backing 112 (e.g., at location 234). Reflected (ie., stray) light 232B adds to the overall reflected signal thereby degrading the contrast (i.e., the ratio of maximum reflectivity to minimum reflectivity) of modulator 224.
The problems caused by stray light in two different types of opto-electronic devices have been discussed above. And it will be understood that stray light causes similar problems in other types of opto-electronic devices as well. Therefore, incorporating a means to capture stray light in opto-electronic devices would improve such devices and, more generally, benefit this art.
In accordance with the present invention, the performance of opto-electronic devices is improved by a metallic anti-mirror. The metallic anti-mirror, which is disposed on a substrate of an opto-electronic device, substantially absorbs stray light that is generated by or received by the opto-electronic device.
In accordance with the illustrative embodiment of the present invention, metallic anti-reflection mirror comprises a first metal layer that is disposed on the substrate of an opto-electronic device, a dielectric layer that is disposed on the first metal layer, and a second metal layer that is disposed on the dielectric layer. This arrangement of layers, when of suitable thickness, creates a cavity that enhances the optical field in second metal layer. While the metal layers can comprise virtually any metal, those that adhere well to the substrate and dielectric layer (e.g., aluminum, chromium, etc.) are advantageously used.
Specific values of the thickness of the first metal layer and the dielectric layer will produce a metallic anti-reflection mirror that completely absorbs light (i.e., has zero reflectivity). These values are dependent on the wavelength of the light and the composition of the materials comprising the various layers. Deviations in thickness will result in an increase in the reflectivity of the anti-reflection mirror. Typically, the first metal layer has a thickness that is in a range between about 100 angstroms and about 400 angstroms and the dielectric layer has a thickness that is in a range between about 750 angstroms to about 4500 angstroms.
In one variation of a metallic anti-reflection mirror in accordance with the illustrative embodiment of the present invention, the second metal layer is partitioned into a first sub-layer and a second sub-layer. The first sub-layer is disposed on the dielectric layer,.and the second sub-layer is disposed on the first sub-layer. The second sub-layer comprises a metal, such as gold or aluminum, that advantageously protects the first sub-layer from oxidation.